This invention relates to a control method for a direct-coupling mechanism in a hydraulic power transmission means of an automatic transmission for automotive vehicles, and more particularly to a control method of controlling the transmission capacity of a direct-coupling mechanism which has input and output members mechanically engageable with each other.
In an automatic vehicle equipped with a hydraulic power transmission means as an automatic transmission, such as a hydraulic torque converter and a hydraulic coupling (hereinafter merely called "the torque converter") in general, the torque converter can provide due to its torque amplifying function a required driving force and a smooth and easy driving feeling over the whole speed region of the vehicle, even with a small number of speed reduction gears provided in the transmission. However, slippage loss inherent in the torque converter can cause degradation in the effective consumption of fuel and an increase in the engine rotational speed by an amount corresponding to the slippage loss, the latter resulting in larger operating noise of the engine.
To avoid this disadvantage, a direct-coupling or locking-up mechanism has been developed and actually brought into use which is adapted to mechanically couple the input and output members of the torque converter together to allow mechanical transmission of all or part of the engine power to the vehicle when the hydraulic power transmission by the torque converter is not necessary.
In order to make best use of the direct-coupling mechanism to improve the power transmission efficiecy and the effective fuel consumption, it has been desired to expand the vehicle speed region wherein the direct-coupling mechanism is operated to a lowest possible value. However, if the mechanical direct-coupling is effected in a low vehicle speed region where also the engine speed is low, it can easily cause large vibrations of the vehicle body as well as large vibration noise due to fluctuations of the engine torque, and also causes a degradation in the driveability.
One way to overcome the above disadvantage would be to control the transmission capacity of the direct-coupling mechanism to vary so as to allow slippage in the direct-coupling mechanism when there occurs certain peak torque fluctuations during operation of the engine in the low vehicle speed region, instead of fully directly coupling the the torque converter. For example, the transmission capacity of the direct-coupling mechanism is variably controlled to an optimum value selected from a plurality of values of the transmission capacity (engaging force) in response to the calculated rotational speed ratio e or slip ratio (1-e) between the input and output members of the torque converter, which are used as feedback values to prevent the rotational speed ratio e from becoming 1 or the slip ratio from becoming 0 in the low vehicle speed region.
However, in incorporating the above method into an actual system, the following problem occurs. For instance, in an embodiment of the invention described later, if the maximum transmission capacity of the direct-coupling mechanism that can be attained by the control system is set at a relatively small value, the control will be smoothly conducted and vibrations and noise of the vehicle body will be mitigated, but at the same time fuel consumption will be increased. If, on the other hand, the maximum transmission capacity is set at a relatively high value, the rotational speed ratio e will sometimes become almost 1, or the slip ratio will approach 0, or the speed ratio e can assume the value 1 momentarily or the slip ratio can assume the value 0 momentarily, whereby the vehicle body vibrates and generates noise.
This is because even by today's electronic control technology the time required to calculate the rotational speed ratio e or the slip ratio, inclusive of the sampling time, is not negligibly short as yet, and also mechanical parts in the feedback control system such as hydraulic devices pose a physical limitation upon the reduction of the response time of the system. Therefore, if the transmission capacity is set at a relatively high value, whereby the speed at which the direct-coupling mechanism is operated toward the direct-coupling position (the speed at which the transmission capacity increases) increases, the control cannot catch up with the speed and, as a result, the rotational speed ratio e or the slip ratio can exceed the upper limit of the respective reference value whereupon the feedback control system now operates to control the transmission capacity so as to decrease, and accordingly the speed ratio e or the slip ratio is caused to decrease below the reference value. When this is repeated, it results in hunting of the torque converter.
Also, it is desirable from the view points of fuel consumption and power transmission efficiency that the driver can select at his will either fuel consumption priority operation mode or output power priority operation and the transmission capacity of the direct-coupling mechanism is controlled in a manner suited to the selected operation mode.